This invention relates generally to an arrangement for moniroting the operating temperature of resistive heating elements, and more particularly to such an arrangement for resistive heating elements used as surface heating units in cooking appliances such as ranges.
In controlling the power applied to a heating element employed as a surface element for a range or cooktop, it is advantageous to know the temperature of the heating element. The conventional approach to monitoring temperature has been to employ a temperature sensor placed beneath the cooking utensil containing the food being heated. This information is used primarily to provide a rapid and reasonably accurate response to changes in the power setting for the heating element. Typically, the sensor is deployed in a closed loop feedback control arrangement with control loop parameters selected to provide rapid response to operator power setting selections. This approach produces satisfactory results; however, such sensors add significant cost to the appliance. Because of this cost, ranges with multiple surface units typically employ, at most, one surface unit with a sensor in a closed loop control system. The other surface units are operated in an open loop mode with no means of taking element temperature into account when controlling element operation. The operator simply makes a power setting selection and the heating element is operated at the corresponding power level regardless of the actual temperatures of the element, utensil or food being cooked.
Another shortcoming of conventional cooking appliances generally presently available is the lack of a means of indicating to the user that the heating element is hot once the element is turned OFF. Typically, an indicator light is provided which is energized when one or more elements are turned ON and deenergized when all elements are turned OFF. The disadvantage of this approach is that a heating element remains hot for some period of time after the element has been turned OFF. For the higher power settings in particular, the element may remain hot for a relatively long time.
An open loop control arrangement for cooking appliances which provides a means of tracking the approximate heating element temperature without use of a relatively expensive temperature sensor in a relatively complex closed loop control arrangement and which would be economically practical for each heating element of a multiple element appliance would be highly desirable. Heating element temperature information provided by such an arrangement could be advantageously employed to alert the user that the heating element is uncomfortably hot to touch as well as to provide more rapid response to power setting changes than conventional open loop controls provide. With the versatility provided in microprocessor based control systems, additional advantageous uses of such temperature information to enhance appliance operation may become feasible.
Accordingly, it is a principal object of the present invention to provide an open loop arrangement for household cooking appliances employing resistive heating elements which provides approximate heating element temperature information without temperature sensor feedback.
It is a further principal object of the present invention to provide an arrangement of the aforementioned type which uses the approximate temperature information to provide a visual indication to the user when the heating element is above a predetermined threshold temperature and which continues to provide the visual indication after the heating element is turned OFF until the temperature has dropped below the threshold temperature.